Mars Express Mission — Imagery in the period 2019-2004
• December 12, 2019: ESA's Mars Express has captured detailed views of the small, scarred and irregularly shaped moon Phobos from different angles during a unique flyby. 1)
- Mars has two moons: Phobos and the smaller and more distant Deimos, named after the Greek mythological personifications of fear (Phobos – hence ‘phobia') and terror (Deimos).
- A new image sequence from Mars Express now captures Phobos' motions and surface in detail. The movie comprises 41 images taken on 17 November 2019, when Phobos passed Mars Express at a distance of 2400 km. Mars Express is currently the only spacecraft capable of close encounters with Phobos.
Figure 1: ESA's Mars Express recently tracked Phobos as the moon passed in front of the spacecraft's camera, capturing detailed views of the small, irregularly shaped body at different angles and stages of the flyby. This sequence comprises 41 images taken by the HRSC on 17 November 2019 during orbit 20,076, when Phobos passed Mars Express at a distance of roughly 2400 km. The images have a resolution of 21 m/pixel. This opportunity allowed the spacecraft to capture many features across the moon's surface; alongside a number of impact craters (including the large and prominent Stickney crater), one can see a number of linear marks and furrows (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
Figure 2: Phobos phase angles explained. This image sequence nicely demonstrates the concept of ‘phase angle': the angle between a light source (in this case, the Sun) and the observer (Mars Express' HRSC), as viewed from the target object itself (Phobos). The initial phase angle is 17 degrees, drops to almost 0 degrees mid-way through (when Phobos is at its brightest), and then rises to 15 degrees by the end of the animation. - Phase angle, and how it corresponds to what we see, is represented in the animation to the right of the image sequence (video credit: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO)
- These data nicely illustrate the concept of ‘phase angle': the angle between a light source (in this case, the Sun) and the observer (Mars Express), as viewed from the target object itself (Phobos). The initial phase angle is 17 degrees, drops to almost 0 degrees mid-way through (when Phobos is at its brightest), and then rises to 15 degrees by the end of the animation.
- To gain a mental image of this trajectory, one can imagine Mars Express observing Phobos from one side, slowly moving across to draw level with it, and then moving away to the other side, drawing an arc in the sky between Phobos and the Sun.
- Images acquired across a range of phase angles, as shown here, are incredibly useful for scientists. Different shadows are cast as the Sun's position changes relative to the target object: this illuminates and highlights the surface features and enables calculations of feature height, depth and relief, and reveals much about the roughness, porosity and reflectivity of the surface material itself.
- A phase angle of zero degrees occurs when the Sun is directly behind the observer. In this alignment, all of the light illuminating Phobos hits the surface vertically and is thus largely reflected back into space, causing the target object to brighten up noticeably, as seen in the animation, and shadows to disappear. The lowest phase angle in this animation is not precisely zero, but 0.92 degrees.
- This arrangement – of the Sun, Mars Express and Phobos where the latter is observed at a phase angle of near zero – is very rare, and happens only three times a year at most. Other chances to achieve a phase angle of under one will not occur until April and September 2020 (in the latter case, Mars Express may achieve a phase angle of precisely zero).
- As such, Mars Express takes every opportunity to view this small and intriguing moon from this angle, to shed light on its properties, behavior, possible origin, orbital characteristics and location in space – and to probe its potential as a mission destination.
- Phobos may be an unfamiliar world, but the phenomenon shown in the movie is familiar to anyone who has seen a full Moon. To create a full Moon, the Sun, Earth and Moon align in a roughly straight line (although, due to orbital inclinations, an exact line-up is rare, and results in a lunar eclipse). Here the phase angle – the angle between the light source (the Sun) and observer on the surface of the Earth, as viewed from the Moon – is zero, just as in the movie of Phobos. Today, 12 December, marks the last full Moon of 2019. So look up, and think of Mars' tiny moon Phobos!
• November 21, 2019: Where the two hemispheres of Mars meet, the planet is covered in broken-up terrain: a sign that slow-but-steady flows of icy material once forged their way through the landscape, carving out a fractured web of valleys, cliffs and isolated mounds of rock. 2)
- Mars is a planet of two halves. Its hemispheres are drastically different; the smooth northern lowlands sit up to three kilometers below the rugged southern highlands, and the surface in Mars' northern regions appears to be far younger than the ancient swaths of the south.
- Where these regions meet, they sometimes form a transition area filled with a wide range of intriguing geological features, patterns and processes: a type of landscape unique to Mars known as fretted terrain. Fretted terrain is found in a couple of key areas on Mars, and an especially good example, named Deuteronilus Mensae, can be seen in these images from Mars Express' High Resolution Stereo Camera (HRSC).
- This landscape shows clear and widespread signs of significant, lasting erosion. As is common with fretted terrain, it contains a mix of cliffs, canyons, scarps, steep-sided and flat-topped mounds (mesa), furrows, fractured ridges and more, a selection of which can be seen dotted across the frame.
- These features were created as flowing material dissected the area, cutting through the existing landscape and carving out a web of winding channels. In the case of Deuteronilus Mensae, flowing ice is the most likely culprit. Scientists believe that this terrain has experienced extensive past glacial activity across numerous martian epochs.
- It is thought that glaciers slowly but surely ate away at the plains and plateaus that once covered this region, leaving only a scattering of steep, flat, isolated mounds of rock in their wake.
- Studies of this region by NASA's Mars Reconnaissance Orbiter have shown that most of the features seen here do indeed contain high levels of water ice. Estimates place the ice content of some glacial features in the region at up to 90%. This suggests that, rather than hosting individual or occasional icy pockets and glaciers, Deuteronilus Mensae may actually represent the remnants of an old regional ice sheet. This ice sheet may once have covered the entire area, lying atop the plateaus and plains. As the martian climate changed this ice began to shift around and disappear, slowly revealing the rock beneath.
- Overall, the features seen in these Mars Express images are reminiscent of the rock- and debris-covered glaciers found in cold regions of Earth. Glaciers may actually be relatively common on both past and present-day Mars; recent studies suggest that the planet may have belts of glacial activity above and below its equator, containing huge amounts of ice covered in thick protective layers of dust, and many other areas show signs of having hosted glaciers in the past – just like Deuteronilus Mensae.
• November 13, 2019: In June, NASA's Curiosity rover reported the highest burst of methane recorded yet, but neither ESA's Mars Express nor the ExoMars Trace Gas Orbiter recorded any signs of the illusive gas, despite flying over the same location at a similar time. 3)
- Methane is of such fascination because on Earth a large proportion is generated by living things. It is known that methane has a lifetime of several hundred years before it is broken down by the Sun's radiation, so the fact that it is detected on Mars suggests it has been released into the atmosphere recently – even if the gas itself was generated billions of years ago.
- The methane mystery on Mars has had many twists and turns in recent years with unexpected detections and non-detections alike. Earlier this year it was reported that ESA's Mars Express had detected a signature that matched one of Curiosity's detections from within Gale Crater.
- A recent spike by Curiosity, measured on 19 June 2019, and the highest yet at 21 ppbv, adds to the mystery because preliminary analysis suggest that Mars Express did not detect any on this occasion. (For comparison, the concentration of methane in Earth's atmosphere is around 1800 ppbv, meaning that for every billion molecules in a given volume, 1800 are methane.)
- The Mars Express measurements were taken in the martian daytime about five hours after Curiosity's nighttime measurements; data collected by Mars Express over one day before also did not reveal any signatures. Meanwhile Curiosity's readings had returned to background levels when further measurements were taken in the following days.
- The Mars Express measurement technique allowing data to be inferred right down to the martian surface with its limit of detection around 2 ppbv.
- The ESA-Roscosmos Trace Gas Orbiter (TGO), the most sensitive detector for trace gases at Mars, also did not detect any methane while flying nearby within a few days before and after Curiosity's detection.
- In general, TGO is capable of measuring at parts per trillion levels and accessing down to about 3 km altitude, but this can depend on how dusty the atmosphere is. When measurements were taken at low latitudes on 21 June 2019, the atmosphere was dusty and cloudy, resulting in measurements accessing 20-15 km above the surface with an upper limit of 0.07 ppbv.
- The global lack of methane recorded by TGO is adding to the overall mystery, and corroborating the results of the different instruments is keeping all teams busy.
• 28 October 2019: Mars Express, ESA's first planetary mission, is a true marathon runner among spacecraft. Launched on 2 June 2003, the spacecraft arrived at Mars during the night of 25 December that same year. On 26 October 2019, this spacecraft completed its twenty-thousandth orbit around Mars. Mars Express is in good company in Martian orbit: NASA's Mars 2001 Odyssey and Mars Reconnaissance Orbiter have also been studying the Red Planet for more than 10 years. Odyssey has been in orbit since 2001 and the Reconnaissance Orbiter since March 2006. 4)
- The HRSC (High Resolution Stereo Camera) developed and built by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in collaboration with German industry, has been photographing the planet's surface on board Mars Express since January 2004 – at resolutions of up to 10 m per pixel, in color and in three dimensions. This is the first global topographic collection of image data for a planet other than Earth. In total, the resistant stereo scanner has recorded 363 GB (GigaByte) of raw data that have been pre-processed on Earth to produce 5.5 GB of scientifically useful image data. The HRSC has recorded 75 percent of the planet's approximately 150 million km2 surface at image resolutions of 10 to 20 m/pixel.
- The topographic image maps generated using the HRSC are of great scientific benefit. Digital HRSC terrain models are also used when selecting landing sites, such as for NASA's InSight geophysical observatory or the ExoMars (ESA, due to launch in 2020), Curiosity and Mars 2020 (NASA) rovers.
- The orbit of Mars Express is highly elliptical, passing from pole to pole and taking the spacecraft to distances between 240 km to over 10,000 km from the Martian surface. The 'anniversary' of the twenty-thousandth orbit gave researchers in the HRSC experiment team, led by Ralf Jaumann at the DLR Institute of Planetary Research in Berlin, the opportunity to visit Mars during a simulated overflight of various 'chaotic areas', outflow channels and craters to the east of the Valles Marineris canyon, just north of the equator.
• 10 October 2019: Mars may seem to be an alien world, but many of its features look eerily familiar – such as this ancient, dried-up river system that stretches out for nearly 700 km across the surface, making it one of the longest valley networks on the planet. 5)
- The area of Mars shown in these new images from ESA's Mars Express spacecraft lies just south of the planet's equator, and is known to have been shaped by a mix of flowing water and impacts: events where rocks sped inwards from space to collide with the martian surface.
- Both of these mechanisms are visible here: a number of impact craters, some large and some small, can be seen speckled across the ochre, caramel-hued surface, and a tree-like, forked channel cuts prominently through the center of the frame.
- This ancient valley system is named Nirgal Vallis, and was once filled with running water that spread across Mars. By exploring the characteristics of the surrounding craters, scientists estimate the system's age to be between 3.5 and 4 billion years old.
- The part of Nirgal Vallis captured in these images lies towards the western end of the river system, where it is slowly spreading out and dissipating; the eastern end is far less branched and more clearly defined as a single valley, and opens out into the large Uzboi Vallis – the suspected location of a large, ancient lake that has long since dried up.
- Nirgal Vallis is a typical example of a feature known as an amphitheatre-headed valley. As the name suggests, rather than ending bluntly or sharply, the ends of these tributaries have the characteristic semi-circular, rounded shape of an Ancient Greek amphitheatre. Such valleys also typically have steep walls, smooth floors, and, if sliced through at a cross-section, adopt a ‘U' shape. The valleys pictured here are about 200 m deep and 2 km wide, and their floors are covered in sandy dunes; the appearance of these dunes indicates that martian winds tend to blow roughly parallel to the valley walls.
- We see valleys like this often on Earth, including valleys found in the Chilean Atacama Desert, the Colorado Plateau, and on the islands of Hawaii. Mars also hosts a few of them, with Nanedi Valles and Echus Chasma joining Nirgal Vallis as clear examples of this intriguing feature. Both of these features also resemble terrestrial drainage systems, where meandering, steep-sided valleys – thought to have been formed by free-flowing water – have carved their way through hundreds of kilometers of martian rock, forging through old volcanic plains, lava flows, and material deposited by strong martian winds over time
- Valleys such as Nirgal Vallis are ubiquitous in the low-latitude regions surrounding the martian equator, indicating that these areas once experienced a far milder and more Earth-like climate. Despite the arid, hostile world we see today, Mars is thought to have once been a far warmer and wetter planet – and we see signs of this in the diverse mix of features and minerals found across its surface.
- Scientists believe that Nirgal Vallis formed in a similar way to morphologically similar valleys we see on Earth. As there appear to be no branching, tree-like tributaries feeding into the main valley of Nirgal Vallis, it is likely that water was replenished on ancient Mars by a mix of precipitation and overland flow from the surrounding terrain.
- The system may also have its roots in a process known as groundwater sapping: when water struggles to travel vertically through a medium, and so instead continually seeps laterally through material in layers beneath the surface. We see this kind of mechanism on Earth in environments where surface material is very fine and loose and thus difficult for water to penetrate – largely silty, sandy, unconsolidated, and fine-grained environments, where lower layers of the surface are permeable and friendlier to water than those above.
- The spacecraft captured these observations using its HRSC, an instrument that is mapping the whole surface Mars in full color and at high resolution. Its aim – of characterizing and understanding the Red Planet in its entirety – will be supported and continued by the ESA-Roscosmos ExoMars Trace Gas Orbiter, which arrived at Mars in 2016, and the ExoMars Rosalind Franklin rover and its accompanying surface science platform, which will arrive next year. Together, this ground-breaking fleet will help unlock the mysteries of Mars.
• 19 September 2019: This beautiful view from ESA's Mars Express stretches from the bright, cloud-covered north pole of Mars to the contrasting hues of the northern hemisphere and the cratered terrain in the south. 6)
- Mars Express has been orbiting Mars since 2003. The spacecraft has sent back myriad breathtaking images of our planetary neighbor in the past decade and a half, captured by the probe's on-board HRSC (High Resolution Stereo Camera) – and this image is no different.
- he spacecraft imaged this slice across the planet's surface in June 2019, when the camera took several global views. Visible at the top of the frame is Mars' ethereal north pole: this is permanently covered by a cap of frozen water and carbon dioxide, which thickens in the northern martian winter and thins in the summer.
- The northern polar cap is seen here encircled by bright, eye-catching clouds, tendrils of which snake downwards from the polar region to obscure some of the planet's northern hemisphere. As this image shows, this patch of Mars is a mix of different tones and colors – a reflection of the different chemical and physical characteristics of the material that makes up the surface. Mars' two hemispheres are very different in a number of ways.
- Most notably, the northern hemisphere sits several kilometers lower than the southern, and the two are separated by a distinctive, rugged boundary formed of canyons, cliffs and scarps, fractures, valleys, flat-topped mounds known as mesas, and many other features. The northern hemisphere is also characterized by low-lying plains that are largely unmarked by impact craters and thus thought to be relatively young, while the southern hemisphere is ancient, showing signs of intense cratering.
- This separation can be seen here, and is shown especially clearly in the accompanying topographic context map.
- The dark and dusty young plains of the northern hemisphere sit just below the white northern cap; these meet and merge with a prominent escarpment that slices across the planet, creating a dark scar on the tan-colored surface. Below this, in tones ranging from rusty orange to pale butterscotch, are the southern highlands, featuring more craters than it is possible to count.
- Two main regions are shown here: Arabia Terra (towards the upper left) and Terra Sabaea (to the middle and lower right, forming the main bulk of the highlands visible in this slice).
- The light region stretching out of frame to the bottom right is Hellas Planitia, a plain that is home to the Hellas basin: one of the largest basins identified on Mars – and, in fact, in the Solar System – at 2300 km across.
- The split between Mars' two hemispheres is known as the martian dichotomy, and remains one of the greatest mysteries about the planet.
- Was it formed due to geological processes within Mars' mantle? Did the planet's crust once comprise various moving tectonic plates, as we see on Earth, that pushed against one another to form the dichotomy? Could it have been created by one or more colossal past impacts – or by another process entirely?
- Observations of the boundary zone between the two hemispheres show that this region has been altered over time by wind and water, including by glaciers. Mars is thought to have seen various bursts of glacial activity over the years, where deposits of ice – sometimes hidden beneath layers of soil or dust – form viscous flows that slowly move across the surface, altering it as they go.
- Mars Express was recently joined at Mars by the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO), which arrived in 2016 and has since been analyzing the martian atmosphere and mapping the planet's surface. Mars Express and the TGO will soon welcome the ExoMars Rosalind Franklin rover and its accompanying surface science platform, which are scheduled for launch in July of 2020.
- This growing fleet will continue ESA's long-standing presence at Mars, and further our understanding of the planet and its many remaining scientific mysteries – including the martian dichotomy.
• 08 August 2019: ESA's Mars Express has captured the cosmic contrast of Terra Cimmeria, a region in the southern highlands of Mars marked by impact craters, water-carved valleys, and sand and dust in numerous chocolate and caramel hues. 7)
- Mars is often referred to as the Red Planet, due to the characteristic hue of its orb in the sky. Up close, however, the planet is actually covered in all manner of colors – from bright whites and dark blacks to yellows, reds, greens, and the cappuccino tones seen here.
- These differences in color are visible from telescopes on Earth. They are undeniably visually striking, but also reveal a significant amount about the composition and properties of the surface material itself.
- These views, based on Mars Express data are a great example of the diversity found on the martian surface: the darker regions towards the right (north) in the image at the top of this page are rich in minerals of volcanic origin, the most common of which found on Mars is basalt. The lighter patches to the left (south) are instead largely covered in fine silicate dust.
- Mars is thought to have once seen significant volcanic activity. The planet hosts some of the largest volcanoes in the Solar System, including the very biggest, Olympus Mons, and has several notable volcanic provinces (two of which are Tharsis and Elysium). The volcanoes within these regions once released ash and dust that covered and coated the surface of Mars, forming dark basaltic sands that were swept around and covered up by other material over time.
- The largest crater in the image measures 25 km across and is 300 m deep – this relatively shallow depth is likely due to the crater being filled up with other material since its formation. Surrounding this crater are various plains, valleys, and ‘mesas' – steep-sided mounds rising up from the martian surface.
- Some of these features are the remnants of a former water-filled valley system, seen most clearly to the upper right of the frame. These valleys spread across Terra Cimmeria, once moving water and material throughout the area.
- This water was locked up within surface ice and snow, but recent research points towards several episodes of melting that unlocked the water from glaciers and sent it flowing across Mars in liquid form.
- To the left of the frame, thin dark trails can be seen snaking and sweeping across Terra Cimmeria – a tell-tale sign that ‘dust devils' were once present here. Dust devils form as eddies of wind that displace the top layer of dust from the martian surface, sending it swirling up into the air. This in turn reveals a deeper layer of material that is different in color, creating a sharp visible contrast.
- Another group of dark, but larger, wind-formed features known as ‘wind streaks' can be seen near the center-left of the image at the top of the page – shown below also as an anaglyph. These form in a similar way to dust devil tracks, except that they are caused not by eddies, but by local winds being forced over topographic features such as craters or cliffs.
- Because of this, streaks can appear to emanate from these features. Wind streaks are useful indicators in atmospheric studies; for instance, the wind that formed the streaks in this image was blowing in a roughly south-easterly direction (given that north is to the right).
- Whether altered by water, wind, impact or other means, the surface of Mars is a dynamic environment – and ESA's Mars Express, in orbit around Mars since 2003, has managed to capture all manner of phenomena on the Red Planet in the past 16 years.
- ESA's fleet at Mars grew with the arrival of the ESA-Roscosmos ExoMars Trace Gas Orbiter in 2016, which has been making a detailed analysis of the planet's atmosphere and mapping its surface. Next year, the ExoMars Rosalind Franklin rover and its accompanying surface science platform will be launched to further our understanding of Mars from the planet's intriguing surface.
• 04 July 2019: ESA's Mars Express has been keeping an eye on local and regional dust storms brewing at the north pole of the Red Planet over the last month, watching as they disperse towards the equator. 8)
- Local and regional storms lasting for a few days or weeks and confined to a small area are common place on Mars, but at their most severe can engulf the entire planet, as experienced last year in a global storm that circled the planet for many months.
- It is currently spring in the northern hemisphere of Mars, and water-ice clouds and small dust-lifting events are frequently observed along the edge of the seasonally retreating ice cap.
- Many of the spacecraft at Mars return daily weather reports from orbit or from the surface, providing global and local impressions of the changing atmospheric conditions. ESA's Mars Express observed at least eight different storms at the edge of the ice cap between 22 May and 10 June, which formed and dissipated very quickly, between one and three days.
- The two cameras onboard the spacecraft, the High Resolution Stereo Camera (HRSC) and the Visual Monitoring Camera (VMC), have been monitoring the storms over the last weeks. The image of Figure 26, taken by HRSC on 26 May, captures a spiral-shaped dust storm, its brown color contrasting against the white ice of the north polar ice cap below.
- Meanwhile the animated sequence (Figure 27) was compiled from images of a different storm captured by the VMC over a period of 70 minutes on 29 May. This particular storm started on 28 May and continued to around 1 June, moving towards the equator during that time.
- The montage of images (Figure 28) shows three different storms developing on 22 May, on 26 May, and between 6 and 10 June. In the latter case, the cameras watched the storm evolve for several days as it moved in an equatorward direction.
- At the same time, wispy patches of light-colored clouds can be seen at the outer margin of the polar cap and also several thousand kilometers away, close to the volcanoes Elysium Mons and Olympus Mons.
- Together with the MARCI camera onboard NASA's Mars Reconnaissance Orbiter, Mars Express observed that when the dust storms reached the large volcanoes, orographic clouds – water ice clouds driven by the influence of the volcano's leeward slope on the air flow – that had previously been developing started to evaporate as a result of the air mass being heated by the influx of dust.
- These regional dust storms only last a few days; the elevated dust is transported and spread out by global circulation into a thin haze in the lower atmosphere, around 20–40 km altitude. Some traces of dust and clouds remained in the volcanic province into mid-June.
• 27 June 2019: The cracked, uneven, jumbled landscape seen in this image from ESA's Mars Express forms an intriguing type of terrain that cannot be found on Earth: chaotic terrain. 9)
- The feature visible here, Aurorae Chaos, is located in the ancient and equatorial Margaritifer Terra region of Mars. The terrain here is heavily cratered, and shows signs of myriad fascinating features – many of which are thought to be linked to past water activity.
- These images show the southern part of Aurorae Chaos in detail, highlighting various swathes of fractured rock, mismatched peaks, flat-topped mounds (mesas), scarps, jumbled cliffs, and eroded craters.
- These characteristic features sweep across the surface, and connect to a number of small outflow channels that spread into this image from beyond the top of the frame in the main color image. These channels form the eastern end of one of Mars' most famous features – a giant valley system called Valles Marineris, which cuts deep into the surface and spans thousands of kilometers.
- This canyon is colossal: about 10 times as long, 20 times as wide, and 4.5 times as deep as Arizona's Grand Canyon here on Earth. The Grand Canyon was carved out by running water, and is thus an excellent example of fluvial erosion – although this kind of erosion is different to that which formed Aurorae Chaos. At its eastern end, the martian canyon runs into a large network of steep-sided depressions that sit roughly four kilometers below the surrounding plains and host numerous chaotic terrains.
- These differences in height are well illustrated in the accompanying topographic, perspective, and 3D views of this region, while the position of Aurorae Chaos with respect to surrounding valleys and chaotic terrain can be seen in the contextual view.
- The division between the chaotic terrain and plains can also be seen clearly in these images. The left (south) side of the image is notably smoother and more featureless than the jumbled right (north) side, and the two regions are split by a prominent line carving diagonally across the frame. The transition area around this scarp is especially broken and fractured; this is thought to be caused as the martian crust stretched and moved.
- The ancient chaotic terrain we see on Mars holds information about how water once permeated and interacted with the planetary surface, including how it was transported, stored, and released.
- Chaotic terrain is thought to have formed as chunks of the martian surface collapsed in dramatic events triggered by the heating of material containing ice or water-bearing minerals – possibly due to climatic or volcanic heat sources, or an impact from an asteroid or comet. This released large amounts of water, causing the terrain above to subside. The water then drained away quickly, leaving behind the messy, broken patterns seen in regions such as Aurorae Chaos, which is thought to have formed some 3.5 billion years ago.
- However, it is not just visual evidence that suggests that water had a large role to play here. The wider region of Margaritifer Terra has been found to contain various sulphates and ancient clay deposits, indicating the past presence of evaporative processes and water-related outflows; some clays are even thought to require standing water in order to form, suggesting that large pools of liquid water may once have existed in this region.
- Over the past 15 years Mars Express has imaged various chaos terrains on Mars, including Iani Chaos and Ariadnes Colles, using its High Resolution Stereo Camera, and continues to study the martian surface from orbit today. Our ability to explore Mars will be aided by the arrival of the ESA-Roscosmos ExoMars rover, named Rosalind Franklin, and an accompanying surface science platform in 2021. Together with the ExoMars Trace Gas Orbiter, which entered Mars orbit in 2016, they will continue our quest to explore the secrets of the Red Planet from orbit and from the ground.
• 16 May 2019: Mars was once believed to be crisscrossed by a system of irrigation canals – dark troughs that sliced across the planet's surface, excavated by an intelligent society of thirsty martians. The astronomer who promoted this idea lends his name to the crater shown in this image from ESA's Mars Express: Lowell crater. 10)11)
- American astronomer and author Percival Lowell is perhaps best known for popularizing this canal theory in the late 1800s and early 1900s; the idea was initially proposed by Italian scientist Giovanni Schiaparelli, who noted the presence of dark lines on Mars in observations from the 1870s. Schiaparelli described these features as canali, later translated not as ‘channels' or ‘gullies', but as ‘canals' – a phrase that hinted at a somewhat more artificial origin.
- Lowell crater is roughly 200 km in diameter and located in a region of Mars known as Aonia Terra, within the planet's ancient southern highlands. The impact that created it is thought to have taken place between 3.7 and 3.9 billion years ago; it has endured erosion and infilling since. Its crater floor has become covered and flattened by various layers of sediment, and its outer rim is marked by small dunes and gullies.
- The image also highlights a ring of mountains rising up from the crater floor and spanning 90 km across. This so-called ‘peak ring' is thought to have formed along with the crater. The immense energy of a large impact event causes material to surge upwards before collapsing down again, forming the kind of complex morphology seen here, with an irregular mountain range encircling the crater's center, inside the main crater rim.
- Such features are also seen in craters here on Earth, and on Venus, Mercury, and the Moon. One notable terrestrial crater that displays this kind of peak ring is Chicxulub crater – famous for its role in the extinction of the dinosaurs some 66 million years ago. Studies and simulations of Chicxulub, which is around the same size as Lowell crater, have shown a peak ring that formed as a huge, unstable central peak later collapsed.
• 13 May 2019: One has a thick poisonous atmosphere, one has hardly any atmosphere at all, and one is just right for life to flourish – but it wasn't always that way. The atmospheres of our two neighbors Venus and Mars can teach us a lot about the past and future scenarios for our own planet. 12)
- Rewind 4.6 billion years from the present day to the planetary construction yard, and we see that all the planets share a common history: they were all born from the same swirling cloud of gas and dust, with the newborn Sun ignited at the center. Slowly but surely, with the help of gravity, dust accumulated into boulders, eventually snowballing into planet-sized entities.
- Rocky material could withstand the heat closest to the Sun, while gassy, icy material could only survive further away, giving rise to the innermost terrestrial planets and the outermost gas and ice giants, respectively. The leftovers made asteroids and comets.
- The atmospheres of the rocky planets were formed as part of the very energetic building process, mostly by outgassing as they cooled down, with some small contributions from volcanic eruptions and minor delivery of water, gases and other ingredients by comets and asteroids. Over time the atmospheres underwent a strong evolution thanks to an intricate combination of factors that ultimately led to the current status, with Earth being the only known planet to support life, and the only one with liquid water on its surface today.
- We know from space missions such as ESA's Venus Express, which observed Venus from orbit between 2006 and 2014, and Mars Express, investigating the Red Planet since 2003, that liquid water once flowed on our sister planets, too. While the water on Venus has long since boiled away, on Mars it is either buried underground or locked up in ice caps. Intimately linked to the story of water – and ultimately to the big question of whether life could have arisen beyond Earth – is the state of a planet's atmosphere. And connected to that, the interplay and exchange of material between the atmosphere, oceans and the planet's rocky interior.
Planetary Recycling
- Back at our newly formed planets, from a ball of molten rock with a mantle surrounding a dense core, they stated to cool down. Earth, Venus and Mars all experienced outgassing activity in these early days, which formed the first young, hot and dense atmospheres. As these atmospheres also cooled, the first oceans rained down from the skies.
- At some stage, though, the characteristics of the geological activity of the three planets diverged. Earth's solid lid cracked into plates, in some places diving below another plate in subduction zones, and in other places colliding to create vast mountain ranges or pulling apart to create giant rifts or new crust. Earth's tectonic plates are still moving today, giving rise to volcanic eruptions or earthquakes at their boundaries.
- Venus, which is only slightly smaller than Earth, may still have volcanic activity today, and its surface seems to have been resurfaced with lavas as recently as half a billion years ago. Today it has no discernible plate tectonics system; its volcanoes were likely powered by thermal plumes rising through the mantle – created in a process that can be likened to a ‘lava lamp' but on a gigantic scale.
- Mars, being a lot smaller, cooled off more quickly than Earth and Venus, and when its volcanoes became extinct it lost a key means of replenishing its atmosphere. But it still boasts the largest volcano in the entire Solar System, the 25 km high Olympus Mons, likely too the result of continuous vertical building of the crust from plumes rising from below. Even though there is evidence for tectonic activity within the last 10 million years, and even the occasional marsquake in present times, the planet is not believed to have an Earth-like tectonics system either.
- It is not just global plate tectonics alone that make Earth special, but the unique combination with oceans. Today our oceans, which cover about two-thirds of Earth's surface, absorb and store much of our planet's heat, transporting it along currents around the globe. As a tectonic plate is dragged down into the mantle, it warms up and releases water and gases trapped in the rocks, which in turn percolate through hydrothermal vents on the ocean floor.
- Extremely hardy lifeforms have been found in such environments at the bottom of Earth's oceans, providing clues as to how early life may have begun, and giving scientists pointers on where to look elsewhere in the Solar System: Jupiter's moon Europa, or Saturn's icy moon Enceladus for example, which conceal oceans of liquid water beneath their icy crusts, with evidence from space missions like Cassini suggesting hydrothermal activity may be present.
- Moreover, plate tectonics helps to modulate our atmosphere, regulating the amount of carbon dioxide on our planet over long timescales. When atmospheric carbon dioxide combines with water, carbonic acid is formed, which in turn dissolves rocks. Rain brings the carbonic acid and calcium to the oceans – carbon dioxide is also dissolved directly in oceans – where it is cycled back into the ocean floor. For almost half of Earth's history the atmosphere contained very little oxygen. Oceanic cyanobacteria were the first to use the Sun's energy to convert carbon dioxide into oxygen, a turning point in providing the atmosphere that much further down the line allowed complex life to flourish. Without the planetary recycling and regulation between the mantle, oceans and atmosphere, Earth may have ended up more like Venus.
Extreme Greenhouse Effect
- Venus is sometimes referred to as Earth's evil twin on account of it being almost the same size but plagued with a thick noxious atmosphere and a sweltering 470ºC surface. Its high pressure and temperature is hot enough to melt lead – and destroy the spacecraft that dare to land on it. Thanks to its dense atmosphere, it is even hotter than planet Mercury, which orbits closer to the Sun. Its dramatic deviation from an Earth-like environment is often used as an example of what happens in a runaway greenhouse effect.
- The main source of heat in the Solar System is the Sun's energy, which warms a planet's surface up, and then the planet radiates energy back into space. An atmosphere traps some of the outgoing energy, retaining heat – the so-called greenhouse effect. It is a natural phenomenon that helps regulate a planet's temperature. If it weren't for greenhouse gases like water vapor, carbon dioxide, methane and ozone, Earth's surface temperature would be about 30 degrees cooler than its present +15ºC average.
- Over the past centuries, humans have altered this natural balance on Earth, strengthening the greenhouse effect since the dawn of industrial activity by contributing additional carbon dioxide along with nitrogen oxides, sulphates and other trace gases and dust and smoke particles into the air. The long-term effects on our planet include global warming, acid rain and the depletion of the ozone layer. The consequences of a warming climate are far-reaching, potentially affecting freshwater resources, global food production and sea level, and triggering an increase in extreme-weather events.
- There is no human activity on Venus, but studying its atmosphere provides a natural laboratory to better understand a runaway greenhouse effect. At some point in its history, Venus began trapping too much heat. It was once thought to host oceans like Earth, but the added heat turned water into steam, and in turn, additional water vapor in the atmosphere trapped more and more heat until entire oceans completely evaporated. Venus Express even showed that water vapor is still escaping from Venus' atmosphere and into space today.
- Venus Express also discovered a mysterious layer of high-altitude sulphur dioxide in the planet's atmosphere. Sulphur dioxide is expected from the emission of volcanoes – over the mission's duration Venus Express recorded large changes in the sulphur dioxide content of the atmosphere. This leads to sulfuric acid clouds and droplets at altitudes of about 50-70 km – any remaining sulphur dioxide should be destroyed by intense solar radiation. So it was a surprise for Venus Express to discover a layer of the gas at around 100 km. It was determined that evaporating sulfuric acid droplets free gaseous sulfuric acid that is then broken apart by sunlight, releasing the sulphur dioxide gas.
- The observation adds to the discussion what might happen if large quantities of sulphur dioxide are injected into Earth's atmosphere – a proposal made for how to mitigate the effects of the changing climate on Earth. The concept was demonstrated from the 1991 volcanic eruption of Mount Pinatubo in the Philippines, when sulphur dioxide ejected from the eruption created small droplets of concentrated sulfuric acid – like those found in Venus' clouds – at about 20 km altitude. This generated a haze layer and cooled our planet globally by about 0.5ºC for several years. Because this haze reflects heat it has been proposed that one way to reduce global temperatures would be to inject artificially large quantities of sulphur dioxide into our atmosphere. However, the natural effects of Mt Pinatubo only offered a temporary cooling effect. Studying the enormous layer of sulfuric acid cloud droplets at Venus offers a natural way to study the longer term effects; an initially protective haze at higher altitude would eventually be converted back into gaseous sulfuric acid, which is transparent and allows all the Sun's rays through. Not to mention the side-effect of acid rain, which on Earth can cause harmful effects on soils, plant life and water.
Global Freezing
- Our other neighbor, Mars, lies at another extreme: although its atmosphere is also predominantly carbon dioxide, today it hardly has any at all, with a total atmospheric volume less than 1% of Earth's.
- Mars' existing atmosphere is so thin that although carbon dioxide condenses into clouds, it cannot retain sufficient energy from the Sun to maintain surface water – it vaporises instantly at the surface. But with its low pressure and relatively balmy temperatures of -55ºC (ranging from -133ºC at the winter pole to +27ºC during summer), spacecraft don't melt on its surface, allowing us greater access to uncover its secrets. Furthermore, thanks to the lack of recycling plate tectonics on the planet, four billion year old rocks are directly accessible to our landers and rovers exploring its surface. Meanwhile our orbiters, including Mars Express, which has been surveying the planet for more than 15 years, are constantly finding evidence for its once flowing waters, oceans and lakes, giving a tantalizing hope that it might have once supported life.
- The Red Planet too would have started out with a thicker atmosphere thanks to the delivery of volatiles from asteroids and comets, and volcanic outgassing from the planet as its rocky interior cooled down. It simply couldn't hold on to its atmosphere most likely because of its smaller mass and lower gravity. In addition, its initial higher temperature would have given more energy to gas molecules in the atmosphere, allowing them to escape more easily. And, having also lost its global magnetic field early in its history, the remaining atmosphere was subsequently exposed to the solar wind – a continuous flow of charged particles from the Sun – that, just as on Venus, continues to strip away the atmosphere even today.
- With a decreased atmosphere, the surface water moved underground, released as vast flash-floods only when impacts heated the ground and released the subsurface water and ice. It is also locked up in the polar ice caps. Mars Express also recently detected a pool of liquid water buried within two kilometers of the surface. Could evidence of life also be underground? This question is at the heart of Europe's ExoMars rover, scheduled to launch in 2020 and land in 2021 to drill up to two meters below the surface to retrieve and analyze samples in search for biomarkers.
- Mars is thought to be currently coming out of an ice age. Like Earth, Mars is sensitive to changes in factors such as the tilt of its rotational axis as it orbits the Sun; it is thought that the stability of water at the surface has varied over thousands to millions of years as the axial tilt of the planet and its distance from the Sun undergo cyclical changes. The ExoMars Trace Gas Orbiter, currently investigating the Red Planet from orbit, recently detected hydrated material in equatorial regions that could represent former locations of the planet's poles in the past.
- The Trace Gas Orbiter's primary mission is to conduct a precise inventory of the planet's atmosphere, in particular the trace gases which make up less than 1% of the planet's total volume of atmosphere. Of particular interest is methane, which on Earth is produced largely by biological activity, and also by natural and geological processes. Hints of methane have previously been reported by Mars Express, and later by NASA's Curiosity rover on the surface of the planet, but the Trace Gas Orbiter's highly sensitive instruments have so far reported a general absence of the gas, deepening the mystery. In order to corroborate the different results, scientists are not only investigating how methane might be created, but also how it might be destroyed close to the surface. Not all lifeforms generate methane, however, and the rover with its underground drill will hopefully be able to tell us more. Certainly the continued exploration of the Red Planet will help us understand how and why Mars' habitability potential has changed over time.
Exploring Farther
- Despite starting with the same ingredients, Earth's neighbors suffered devastating climate catastrophes and could not hold on to their water for long. Venus became too hot and Mars too cold; only Earth became the ‘Goldilocks' planet with the just-right conditions. Did we come close to becoming Mars-like in a previous ice age? How close are we to the runaway greenhouse effect that plagues Venus? Understanding the evolution of these planets and the role of their atmospheres is tremendously important for understanding climatic changes on our own planet as ultimately the same laws of physics govern all. The data returned from our orbiting spacecraft provide natural reminders that climate stability is not something to be taken for granted.
- In any case, in the very long term – billions of years into the future – a greenhouse Earth is an inevitable outcome at the hands of the aging Sun. Our once life-giving star will eventually swell and brighten, injecting enough heat into Earth's delicate system to boil our oceans, sending it down the same pathway as its evil twin.
- We are changing our natural world faster than at any other time in history. Understanding the intricacies of how Earth works as a system and the impact that human activity is having on natural processes are huge environmental challenges. Satellites are vital for taking the pulse of our planet, delivering the information we need to understand and monitor our precious world, and for making decisions to safeguard our future. Earth observation data is also key to a myriad of practical applications to improve everyday life and to boost economies. This week we focus on the world's biggest conference on Earth observation where thousands of scientists and data users discuss the latest results and look to the future of Earth observation (Ref. 12) .
• 08 April 2019: Bacterial resistance to antibiotics is one of humankind's major long-term health challenges. Now research into helping humans live on Mars could help address this looming problem. 14)
- Dennis Claessen, associate professor at the Institute of Biology at Leiden University, the Netherlands works in synthetic biology, in which bacteria are engineered to solve problems that cannot be tackled – or are not tackled well – by ‘wild' bacteria. A team of his students entered the iGEM (International Genetically Engineered Machine) competition with a solution to the problem of growing non-toxic plants on Mars, but needed ‘Martian' gravity to test their ideas.
- "The soil on Mars has perchlorate chemical compounds in it, which can be toxic for humans," explains Prof. Claessen. High doses of perchlorate can inhibit the thyroid gland's uptake of iodine and interfere with foetal development. - Our students started ‘building' a bacterium that would degrade the perchlorate to chlorine and oxygen, but they needed to know whether that bacterium would behave the same way in the partial gravity of Mars as it would on Earth."
Figure 41: The RPM (Random Positioning Machine) was developed by the Dutch office of Airbus for ESA to do experiments in zero or reduced gravity here on Earth without going into space. The RPM rotates any enclosed experiment ‘randomly' to minimize the influence of Earth's gravity, thereby simulating what would be experienced in space. The original models could successfully simulate zero gravity, typically referred to as microgravity. The newer RPM 2.0 can additionally simulate partial gravity, which is between 0g and 1g (video credit: Airbus, Released on 5 April 2019)
- The challenge was to find a way to reproduce Mars gravity on Earth, and the students solved it using the RPM.
- The first recorded experiment on living systems using machines to manipulate gravity was done in 1806 using a rotating waterwheel. Two hundred years later the RPM, designed by the Netherlands-based Airbus team for ESA, is the latest instrument developed to experiment in zero or reduced gravity without going into space.
- As its name suggests, the RPM continually changes its orientation at random, so that items placed within it have no opportunity to adjust to a steady gravity direction. The original design could successfully simulate zero gravity while the newer RPM 2.0 can additionally simulate partial gravity, the stages between normal Earth gravity and the weightless environment.
- "The RPM machines offer a great alternative to organizations looking to do experiments in zero and partial gravity," says Derk Schneemann at Verhaert Netherlands. Derk is the Dutch broker in the network of ESA Innovation Partners that facilitates the re-use of space technology in other sectors and which is part of ESA's Business Applications and Space Solutions Program.
- "During their experiments they noticed that when bacteria grew in partial gravity, they became stressed as they accumulated waste around them that they couldn't get rid of. This holds great potential because when microbes belonging to the Streptomyces family become stressed, they usually start making antibiotics.
- "Seventy percent of all the antibiotics humans use are derived from Streptomyces bacteria and we know they have the potential to produce even more. Using the RPM to stress them in new ways may help us to find ones we've never seen before."
Detoxifying Soil on Mars and Earth
- Prof. Claessen is now building a Dutch consortium to investigate soil detoxification on a larger scale. This will have applications on Earth. For example, there is a lot of perchlorate in Chile's Atacama Desert, where the soil is believed to resemble that on Mars. Atacama soil was previously used as fertilizer in the US, but later it was found that perchlorate had been washed from this fertilizer into groundwater used for drinking.
- Once the Dutch team is in place and has funding, research into Streptomyces microbes (which are naturally found in Earth's soil, where they play a vital role in breaking down organic matter) and antibiotics could also be done with the RPM.
- "To find that they hold the potential for discovering new antibiotics as well is even more exciting, as antibiotic resistance is something we need to tackle urgently," adds Derk Schneemann.
Space for Health
- The United Nations World Health Organization marks World Health Day on 7 April every year. The third Sustainable Development Goal underlines the right to health: Ensure healthy lives and promote well-being for all at all ages. On-orbit research, space technology and space applications can help improve health on Earth by monitoring our environment, helping track disease, improving diagnostics, and working on new medicines among other things. The UN is also focusing particularly this year on universal health coverage.
• 01 April 2019: A reanalysis of data collected by ESA's Mars Express during the first 20 months of NASA's Curiosity mission found one case of correlated methane detection, the first time an in-situ measurement has been independently confirmed from orbit. 15)
- Reports of methane in the martian atmosphere have been intensely debated, with Mars Express contributing one of the first measurements from orbit in 2004, shortly after its arrival at the Red Planet.
- The molecule attracts such attention because on Earth methane is generated by living organisms, as well as geological processes. Because it can be destroyed quickly by atmospheric processes, any detection of the molecule in the martian atmosphere means it must have been released relatively recently – even if the methane itself was produced millions or billions of years ago and lay trapped in underground reservoirs until now.
Legend to Figure 44: The details of the methane spike are illustrated in this graphic – the Mars Express detection was made one day after the elevated reading recorded by Curiosity, which is exploring Gale Crater, just south of the martian equator. Taken together, the two results can be used to examine the possible source region for the methane.
Two independent analyses were made, by examining a wide region around Gale Crater. The region was divided into grids of about 250 x 250 km2, and in one study, computer simulations were used to predict the probability of methane emission from each square (indicated by the numbers in each square). The simulations took into account the measured data, expected atmospheric circulation patterns, and methane release intensity and duration based on the geological phenomenon of ‘gas seepage'. In the other study, geologists scrutinized the region for features where gas seepage is expected – these are the kind of features that might be associated with methane release.
The geological analysis pointed to one of the regions that the computer simulations predicted would be the most likely region for methane release. The area marked by the black dots is thought to contain shallow ice that could easily trap subsurface methane, and tectonic faults in the grid square between this region and Gale Crater are thought to extend below the surface and break through the ice, causing the episodic release of methane.
- While spacecraft and telescopic observations from Earth have in general reported no or very low detections of methane, or measurements right at the limit of the instruments' capabilities, a handful of spurious spikes, along with Curiosity's reported seasonal variation at its location in Gale Crater, raise the exciting question of how it is being generated and destroyed in present times.
- Now, for the first time, a strong signal measured by the Curiosity rover on 15 June 2013 is backed up by an independent observation by the Planetary Fourier Spectrometer (PFS) onboard Mars Express the next day, as the spacecraft flew over Gale Crater.
- The study exploited a new observation technique, allowing the collection of several hundred measurements in one area over a short period of time. The teams also developed a refined analysis technique to get the best out of their data.
- "In general we did not detect any methane, aside from one definite detection of about 15 parts per billion by volume of methane in the atmosphere, which turned out to be a day after Curiosity reported a spike of about six parts per billion," says Marco Giuranna from the Institute for Space Astrophysics and Planetology in Rome, Italy, the principal investigator for the PFS experiment, and lead author of the paper reporting the results in Nature Geoscience today. 16)
- "Although parts per billion in general means a relatively small amount, it is quite remarkable for Mars – our measurement corresponds to an average of about 46 tons of methane that was present in the area of 49,000 km2 observed from our orbit."
- Ten other observations in the Mars Express study period that reported no detections at the limit of the spectrometer's sensitivity corresponded to a period of low measurements reported by Curiosity.
Pinpointing the Source
- At the time of the Curiosity detection, it was speculated that the methane originated north of the rover, because the prevailing winds were southward, and that the release likely occurred inside the crater.
- "Our new Mars Express data, taken one day after Curiosity's recording, change the interpretation of where the methane originated from, especially when considering global atmospheric circulation patterns together with the local geology," adds Marco. "Based on geological evidence and the amount of methane that we measured, we think that the source is unlikely to be located within the crater."
- Marco and his colleagues made two independent analyses to home in on potential source regions of the methane, dividing up a wide region around Gale Crater into grids of about 250 x 250 km2.
- In one study, collaborators from the Royal Belgian Institute for Space Aeronomy in Brussels applied computer simulations to create one million emission scenarios for each square, in order to predict the probability of methane emission for each of those locations. The simulations took into account the measured data, expected atmospheric circulation patterns, and methane release intensity and duration based on the geological phenomenon of ‘gas seepage'.
- In the other parallel study, geologists from the National Institute of Geophysics and Volcanology in Rome, Italy and the Planetary Science Institute in Tucson, Arizona, scrutinized the region around Gale Crater for features where gas seepage is expected – these are the kind of features that might be associated with methane release.
- This process is well known on Earth to occur along tectonic faults and from natural gas fields, with a variety of release intensities. For example, on Earth, gas emission from active mud volcanoes is typically continuous with background variations, but also with sudden strong bursts, while other seeps might release gas intermittently.
- Episodic gas release, that is, generally long quiescence with no emission in between short-duration bursts, is typical of the expulsion of gas from small or ‘dying' seeps or due to seismic events. On Mars, episodic gas expulsions could also be created during a meteorite impact, liberating gas trapped below the surface.
- "We identified tectonic faults that might extend below a region proposed to contain shallow ice. Since permafrost is an excellent seal for methane, it is possible that the ice here could trap subsurface methane and release it episodically along the faults that break through this ice," says co-author Giuseppe Etiope from the National Institute of Geophysics and Volcanology in Rome.
- "Remarkably, we saw that the atmospheric simulation and geological assessment, performed independently of each other, suggested the same region of provenance of the methane."
- "Our results support the idea that methane release on Mars might be characterized by small, transient geological events rather than a constantly replenishing global presence, but we also need to understand better how methane is removed from the atmosphere, and how to reconcile the Mars Express data with results from other missions," adds co-author Frank Daerden from the Royal Belgian Institute for Space Aeronomy in Brussels.
- "We will re-analyze more of the data collected by our instrument in the past, while continuing our ongoing monitoring efforts, including coordinating some observations with the ExoMars Trace Gas Orbiter," concludes Marco.
- The ESA-Roscosmos ExoMars Trace Gas Orbiter, which is designed to make the most detailed inventory of the martian atmosphere yet, began its science observations in April 2018.
- "Mars Express was the first to report a significant detection of methane from orbit around Mars, and now, fifteen years later, we can announce the first simultaneous and co-located detection of methane with a rover on the surface," says Dmitri Titov, ESA's Mars Express project scientist.
- "With the spacecraft and its payload still operative, Mars Express is one of the most successful space missions to be sent to Earth's planetary neighbor. We expect more exciting science from joint efforts by both ESA orbiters at Mars."
- The Mars Express detections were made by the Planetary Fourier Spectrometer (PFS). The Curiosity measurements were made by the Tunable Laser Spectrometer – Sample Analysis at Mars instrument (TLS-SAM).
Legend to Figure 45: Methane can also be generated by reactions between carbon dioxide and hydrogen (which, in turn, can be produced by reaction of water and olivine-rich rocks), by deep magmatic degassing or by thermal degradation of ancient organic matter. Again, this could be stored underground and outgassed through cracks in the surface. Methane can also become trapped in pockets of shallow ice, such as seasonal permafrost.
Ultraviolet radiation can both generate methane – through reactions with other molecules or organic material already on the surface, such as comet dust falling onto Mars – and break it down. Ultraviolet reactions in the upper atmosphere (above 60 km) and oxidation reactions in the lower atmosphere (below 60 km) acts to transform methane into carbon dioxide, hydrogen and water vapor, and leads to a lifetime of the molecule of about 300 years.
Methane can also be quickly distributed around the planet by atmospheric circulation, diluting its signal and making it challenging to identify individual sources. Because of the lifetime of the molecule when considering atmospheric processes, any detections today imply it has been released relatively recently.
But other generation and destruction methods have been proposed which explain more localized detections and also allow a faster removal of methane from the atmosphere, closer to the surface of the planet. Dust is abundant in the lower atmosphere below 10 km and may play a role, along with interactions directly with the surface. For example, one idea is that methane diffuses or ‘seeps' through the surface in localized regions, and is adsorbed back into the surface regolith. Another idea is that strong winds eroding the planet's surface allows methane to react quickly with dust grains, removing the signature of methane. Seasonal dust storms and dust devils could also accelerate this process.
Continued exploration at Mars – from orbit and the surface alike – along with laboratory experiments and simulations, will help scientists to better understand the different processes involved in generating and destroying methane.
• 28 March 2019: The winds of Mars are responsible for myriad features across the planet's surface – including the dark dunes and wispy, filament-like streaks seen in this image from ESA's Mars Express. 17)
- The intriguing features shown here are ‘dust devil' tracks: as the Sun heats up the martian ground during the day, vortices form that lift warm air from near the surface, whipping up dust as they do so, shaping and sculpting it into swirling, column-shaped, tornado-like whirlwinds (click here for videos of dust devils made by NASA's Mars rover Spirit).
- These dust devils range across the entire planet, lifting the top, brighter layer of dust from the surface, and leaving darker paths in their wake. They are most often seen in the martian spring and summer, lasting for a few months at most before their tracks become obscured by dust that has been buffeted around by storms and winds.
- These Mars Express images show a curving, looping, crisscrossing web of dust devil tracks in the southern hemisphere of the planet, around an escarpment feature known as Chalcoporos Rupes. This area is covered in a thick layer of dust and is not infrequently home to wind-related activity.
- Areas of Mars that most regularly see dust devils include Amazonis Planitia, Argyre Planitia, Hellas Basin, and two impact craters that lie close to the region shown here: Proctor and Russell.
- Proctor, Russell, and Chalcoproros Rupes are based in Mars' Noachis quadrangle, an area so thickly pockmarked with impact craters that it is thought to be one of the oldest parts of the planet.
- Both the craters visible in this frame boast dense, dark, eye-catching patches of rippling sand dunes, while the surrounding terrain is decorated with a broad web of dunes and signs of past dust devil activity.
- Martian dust devils are similar to those seen on Earth in especially dry, arid, desert landscapes – but they are far larger. They can tower up to eight kilometers high on the Red Planet, creating paths that are hundreds of meters wide and stretch out for a few kilometers.
- Their colossal size makes them highly effective at carrying dust high up into Mars' atmosphere – in fact, these devils may lift as much material as a martian global dust storm does at its peak.
- Such dust storms are immense and impressive. Mars Express captured signs of a burgeoning storm near Mars' north pole in April of last year, highlighting an intense boundary between the planet's usual, calm, ochre-hued surface and an incoming wall of dust clouds – and this was a somewhat modest dust storm compared to those that blanket the entirety of Mars and rage on for months.
- Dust devils have been seen often on Mars, both by Mars Express and other missions – including the ESA-Roscosmos ExoMars Trace Gas Orbiter, which recently imaged an impressive pattern of dust devil tracks in the Terra Sabaea region of Mars that may be the result of hundreds or even thousands of small martian tornadoes coming together and leaving their mark on the planet's surface.
- The Trace Gas Orbiter will be joined by a rover – recently named Rosalind Franklin – and a surface science platform, due to launch in 2020. These will allow the ExoMars mission to explore the Red Planet in even greater detail in coming years.
• 28 February 2019: Mars Express has revealed the first geological evidence of a system of ancient interconnected lakes that once lay deep beneath the Red Planet's surface, five of which may contain minerals crucial to life. 18)